The process of cooking in a conventional gas or electric oven is relatively uncomplicated. Generally, temperature and time are the only two cooking parameters considered. Normally, the oven is preheated to a given temperature and the food is placed in the oven for a specified time period which is sometimes determined by the weight of the food. For example, it may be preferable to cook a turkey at 350.degree. F. for 20 minutes per pound. Generally speaking, the heat at the surface of the food gradually travels inward by conduction raising the temperature of the interior and causing physical changes which are part of the cooking process. Because this cooking process is relatively slow and is always limited by the temperature of the oven so that there can be no thermal runaway, there is a reasonable tolerance in the selection of the cooking parameters. For example, a deviation of 10 minutes per hour or 25.degree. F. in temperature may not have a significant impact on the palatability of the cooked food. This tolerance has contributed to a general confidence of most cooks of their ability to accurately select temperature and time, even in new situations. Another contributing factor is exposure in that most cooks grew up in homes where all of the cooking was done in conventional gas or electric ovens.
The microwave oven has evolved in the last two or three decades. Although consumer acceptance has greatly increased as has the percentage of households with microwave ovens, some consumers are still reluctant to buy or use microwave ovens because they don't have the general confidence in their ability to operate them; they feel intimidated by the sometimes complicated directions for using them. They no longer have the comfortable parameters of temperature and time to select.
The introduction or indoctrination into a relatively new cooking process is complicated by the rate at which foods cook. More specifically, because a microwave oven cooks so fast, an error of a few minutes in the selected cooking time can be a substantial percentage of the required cooking time and can result in a substantial difference in the doneness of the food. Furthermore, the temperature of the food body is not limited by the temperature of the oven; temperature runaway can occur. Accordingly, microwave oven manufacturers have expended considerable effort in research and development of apparatus and methods for simplifying the user task of determining the cooking parameters for microwave ovens. Simplified user operation would presumably expand the consumer marketplace.
One prior art approach is to provide a temperature probe which the user inserts in the food body. The oven is then permitted to remain on until the internal temperature rises to a selected value. This has been accomplished at a predetermined microwave power level set by the user. Once set, the microwave generating system operates at the chosen power level, which is reflective of a particular duty cycle for a magnetron, until the food is cooked or the power level is changed by the user. Setting the microwave power level requires a thorough knowledge of the characteristics of microwave cooking and the cooking abilities of the particular microwave oven used. There are many brands of microwave ovens with a multitude of maximum cooking powers, cooking characteristics and types of controls. All these add to the likelihood of a user error in selecting the proper power level.
The difficulty associated with the selection of the proper microwave power level is compounded by the nature of the food to be cooked and the cooking process itself. All foods are different, and they change as they cook. Different foods and different amounts of the same foods cook better at different power levels. Further, as the cooking process proceeds, the nature of the foods changes causing changes in the foods' ability to absorb the microwave energy. Hence, the optimum power level for starting the cooking process may not be the best for finishing it. Too high of a power level may overcook the food. Too low of one may undercook it or take an unnecessarily long period of time to cook the food satisfactorily.
Traditional radiant and circulated hot air ovens rely primarily on heat conduction from the surface of the food for cooking. Microwave ovens, on the other hand, generate microwave energy which penetrates the surface of the food a certain depth before being completely absorbed by the food. After that, however, even microwave ovens rely on heat conduction to cook the center of many thicker foods. In this instance in particular, there is a distinct possibility that the surface of the food may overcook before the center is cooked or the center may be left undercooked to preserve the appearance and quality of the surface of the food.
If the surface of the food, in the case of some foods, and the center of the food, in the case of other foods, could be held at a desired temperature, the cooking process could proceed while minimizing the possibility of overcooking or undercooking. The present invention accomplishes that while eliminating the possibility of user error in setting the power level by automatically reducing the microwave power level as the temperature of the food rises. At the same time, the power not consumed by the microwave generating system may be utilized by a radiant or forced hot air heater to increase the browning and crisping as the food reaches the desired degree of doneness.